Frequency domain (FD) sensor is an instrument developed for measuring soil moisture content. The instrument has an oscillating circuit, the sensing part of the sensor is embedded in the soil, and the operating frequency will depend on the value of soil's dielectric constant.
Types of sensorsedit
Capacitance probe, or fringe capacitance sensor. Capacitance probes use capacitance to measure the dielectric permittivity of the soil. The volume of water in the total volume of soil most heavily influences the dielectric permittivity of the soil because the dielectric constant of water (80) is much greater than the other constituents of the soil (mineral soil: 4, organic matter: 4, air: 1). Thus, when the amount of water changes in the soil, the probe will measure a change in capacitance (from the change in dielectric permittivity) that can be directly correlated with a change in water content. Circuitry inside some commercial probes change the capacitance measurement into a proportional millivolt output. Other configuration are like the neutron probe where an access tube made of PVC is installed in the soil. The probe consists of sensing head at fixed depth. The sensing head consists of an oscillator circuit, the frequency is determined by an annular electrode, fringe-effect capacitor, and the dielectric constant of the soil.
Electrical impedance sensor, which consists of soil probes and using electrical impedance measurement. The most common configuration is based on the standing wave principle (Gaskin & Miller, 1996). The device comprises a 100 MHz sinusoidal oscillator, a fixed impedance coaxialtransmission line, and probe wires which is buried in the soil. The oscillator signal is propagated along the transmission line into the soil probe, and if the probe's impedance differs from that of the transmission line, a proportion of the incident signal is reflected back along the line towards the signal source.
Benefits and limitationsedit
Compared to time domain reflectometer (TDR), FD sensors are cheaper to build and have a faster response time. However, because of the complex electrical field around the probe, the sensor needs to be calibrated for different soil types. Some commercial sensors have been able to remove the soil type sensitivity by using a high frequency.
Referencesedit
Gaskin G.J., Miller J.D. 1996. Measurement of soil water content using a simplified impedance measuring technique. Journal of Agricultural Engineering Research 63, 153-160.
Campbell C.S., Campbell G.S., Cobos D.R.2004. Response of Low Cost Dielectric Moisture Sensor to Temperature Variation. Eos Trans. AGU, 85(17), Jt. Assem. Suppl. Abstract NS44A-05.
March 17, 2024
frequency, domain, sensor, frequency, domain, sensor, instrument, developed, measuring, soil, moisture, content, instrument, oscillating, circuit, sensing, part, sensor, embedded, soil, operating, frequency, will, depend, value, soil, dielectric, constant, typ. Frequency domain FD sensor is an instrument developed for measuring soil moisture content The instrument has an oscillating circuit the sensing part of the sensor is embedded in the soil and the operating frequency will depend on the value of soil s dielectric constant Types of sensors editCapacitance probe or fringe capacitance sensor Capacitance probes use capacitance to measure the dielectric permittivity of the soil The volume of water in the total volume of soil most heavily influences the dielectric permittivity of the soil because the dielectric constant of water 80 is much greater than the other constituents of the soil mineral soil 4 organic matter 4 air 1 Thus when the amount of water changes in the soil the probe will measure a change in capacitance from the change in dielectric permittivity that can be directly correlated with a change in water content Circuitry inside some commercial probes change the capacitance measurement into a proportional millivolt output Other configuration are like the neutron probe where an access tube made of PVC is installed in the soil The probe consists of sensing head at fixed depth The sensing head consists of an oscillator circuit the frequency is determined by an annular electrode fringe effect capacitor and the dielectric constant of the soil Electrical impedance sensor which consists of soil probes and using electrical impedance measurement The most common configuration is based on the standing wave principle Gaskin amp Miller 1996 The device comprises a 100 MHz sinusoidal oscillator a fixed impedance coaxial transmission line and probe wires which is buried in the soil The oscillator signal is propagated along the transmission line into the soil probe and if the probe s impedance differs from that of the transmission line a proportion of the incident signal is reflected back along the line towards the signal source Benefits and limitations editCompared to time domain reflectometer TDR FD sensors are cheaper to build and have a faster response time However because of the complex electrical field around the probe the sensor needs to be calibrated for different soil types Some commercial sensors have been able to remove the soil type sensitivity by using a high frequency References editGaskin G J Miller J D 1996 Measurement of soil water content using a simplified impedance measuring technique Journal of Agricultural Engineering Research 63 153 160 Campbell C S Campbell G S Cobos D R 2004 Response of Low Cost Dielectric Moisture Sensor to Temperature Variation Eos Trans AGU 85 17 Jt Assem Suppl Abstract NS44A 05 Retrieved from https en wikipedia org w index php title Frequency domain sensor amp oldid 1123460650, wikipedia, wiki, book, books, library,
